Masked plate xerography



H. E. CLARK ETAL MASKED PLATE XEROGRAPHY June 14, 1966 Original Filed Aug. 11, 1961 2 Sheets-Sheet 1 HAROLD E. CLARK ROBERT W. GUNDLACH 'MORTIM R LEVY June 14, 1966 H. E. CLARK ETAL 3,256,089

MASKED PLATE XEROGRAPHY Original Filed Aug. l1, 1961 2 Sheets-Sheet 2 INVENTORS.

HAROLD E. CLARK ROBERT W. GUNDLACH MORTIME LEVY United States Patent O This application is a division of our application, Serial No. 130,965, led August ll, 1961, now U.S. Patent No. 3,182,573.

This application relates to xerography and more particularly to an improved form of xerographic pl-ate and to methods and apparatus for use therewith.

In the most common form of xerography an electrostatic latent image is formed on a photoconductive insulating layer of xerographic plate by uniformly electrostatically charging the photoconductive insulating layer and then selectively discharging the layer through exposure to a pattern of light and shadow. The electrostatic latent image thus formed can be developed or made visible through the selective electrostatic attraction of pigmented particles or the like -and the developed image may be viewed on the xerographic plate or else transferred toa sheet of paper or other suitable support. Various forms of photoconductive insulating layers are known in the art of xerography and are pertinent to the present invention. These include but are not limited to amorphous materials such as anthracene or vitreous selenium and paint-like films comprising photoconductive pigments such as zinc oxide and an electrically insulating film forming binder. Such photoconductive insulating materials may in certain circumstances be employed as self supporting films but the xerographic plate of commerce generally employs a photoconductive insulating material on a suitable support -such as a metal plate, a sheet of paper or the like.

Depending upon the character of the support layer the plate may take various forms including rigid tlat plates, llexible webs, cylinders and the like. It is generally characteristic of such plates that the support layer is either opaque or of a highly light scattering nature. Accordingly with conventional plates the steps of charging the plate,l exposing the plate tov a pattern of light and shadow, and developing the resulting electrostatic latent image are all carried out on that side of the plate carrying the photoconductive insulating layer. Xerographic pla-tes are also known, however, in which the photoconductive insulating layer -is carried on a supporting layer which comprises a sheet of glass or other transparent material. Such plates can be processed in the conventional manner except that image exposure may 'be effected through the transparent support layer rather than directly onto the photoconductive insulating layer. Such xerographic plates have greatly enlarged the iiexibility of the xerographic process and have facilitated the creation of new and useful forms of xerographic apparatus Such plates, however, suler from the disadvantage of being sensitive to light at both surfaces. This characteristic causes serious drawbacks in designing simple machines for use with such transparently backed plates and makes their use nearly impossible in normal types of procedures which are, for obviousreasons, preferably carried out `under ambient light conditions.

In accordance with the present invention, however, there is provided a novel form of xerographic plate which can bev char-ged and developed by completely conventional techniques, and on which an electrostatic latent image can be formed by exposure to a pattern of light yand shadow through a transparent support layer but which is substantially insensitive to illumination applied directly to the photoconductive insulating layer. In accordance with the present invention there are also provided novel apparatus and methods employing the novel xerographic plates of the present invention.

Further description of the invention will be made in conjunction wit-h the drawings in which:

FIG. 1 is -a cross sectional View of a plate according to the invention; i

FIG. 2 is a schematic view illustrating one method of making a plate according to the invention;

FIG. 3 illustrates the utilization of a plate according to the invention;

FIG. 4 represents a form of copying apparatus according to the invention; and, i

FIG. 5 represents a modification of the apparatus of FIG. 4. l

FIG. 1 illustrates the structure of a xerographic plate it) made according to ithe present invention. Reference numeral 11 represents a transparent support layer for the plate. This layer preferably comprises a sheet of glass but may also comprise various forms of plastic sheet or other transparent materials. Inasmuch as layer 11 will generally be an excellent electric insulator it is desirable, in accord-ance with conventional xerographic practice, to provide a transparent electrically conductive layer 12 on transparent support layer 11. When layer 11 is of glass layer 12. will generally be an extremely thin layer of tinoxide. It will be appreciated that layer 12 as well as the other layers in this figure are shown schematically and are not representative of the true relative thickness of the various layers. Gl-a-ss sheets with a transparent conductive tin oxide coating are commercially available under the tradenames Nesa and Electropane Where layer 11 is a plastic tin oxide is not generally suitable for layer 12 since its application requires very high temperatures. Accordingly other conductive coatings such as copper iodide may be employed. Coated on top of transparent conduct-ive layer 12 is a layer 13 of photoconductive insulating material. This may be of the types already described or or any other type generally known and employed in the art of xerography. Finally there isapplied over the photoconductive insulating layer 13 a light desensitiz-ing layer 14.

Plate ltlis shown for illustrative purposes as being mounted in a plate holder 15 equipped with a dark slide 16. IPlate holder 15 resembles the plate and tilm holders used in photography and is substantially identical wi-th the plate holder widely used in commercial xerography except that dark slide 16 when in position `blocks light from reaching support layer 11 rather than photoconductive insulating layer 13.

Light desensitizing layer 14 may be either a layer which is substantially opaque to actinic light or else it may be a semi-conductive layer having certain special properties which will be described subsequently. While it would appear simple to devise a suitable opaque overcoating for the photoconductive insulating layer of a xerographic plate such has not 'been the case. The overcoating must not only be -substantially opaque at those wave lengths which impart conductivity to the photoconductive insulating l-ayer but it must also be sufficiently thin and have sufciently excellent dielectric properties to permit the xero- 4graphic plate to accept and retain an electrostatic charge and also to permit the formation and retention of an electrostatic latent image in response to illumination of the plate. It has, however, been found possible to meet these requirements through the use of a layer 14 comprising a suit-able dye dis-solved in a suitable resinous lm forming material.

FIG. 2 represents a form of apparatus suitable for applying an opaque oyercoating to a xerographic plate.

lThe apparatus includes a container 20 lled with a coating liquid 21 and adapted to contain a xerographic plate suspended by a string 22 which is wound upon a drum 23 which in turn is driven at low speed by a motor 24. In operation plate 10 is immersed in container 20 `and slowly withdrawn therefrom thus forming a thin uniform coating on plate 10. A suitable coating liquid for use in this apparatus comprises three liters of ethylene dichloride in which is dissolved 50 grams of the dye known as nigrosine base 1630 and 300 grams of cellulose acetate butyrate such as Eastman Kodak CAB-171-2. A xerographic plate comprising an approximately micron thick layer of vitreous selenium deposited upon a -tin oxide coated glass plate was immersed in and withdrawn from the above described coating liquid at a rate of 1/3 per second thereby forming on the plate an overcoating layer approximately 7 microns thick. By drying and recoating several times it is possible to form a substantially opaque layer having a thickness of about microns. This procedure also forms an overcoating layer on the transparent support layer 11 as well as on the photoconductive insulating layer. The undesired layer formed on the support layer can be readily removed however by wiping with a clothsaturated with ethylene dichloride. The particular coating and method shown in FIG. 2 is well adapted to the present invention but other known coating methods such as spraying or ow coating may also be used.

Xerographic plates made by the above procedure can readily accept an initial electrostatic charge of 400 Volts and are substantially insensitive to light directed through layer 14, an exposure of at least several thousand foot candle seconds being required to dissipate the charge on the plate. The plates are however normally sensitive to lighted directed through transparent support layer 11 and are capable of forming a developable electrostatic latent image when so exposed to a pattern of light and shadow. When thicker overcoatings such as, for example, coatings greater than 25 microns in thickness were applied it was found that the plates could be electrostatically charged without difHculty but this charge could not be dissipated by exposure to light at either surface of the plate, thus making such plates useless in xerography.

Light desensitizing layer 14 may also comprise a thin semi-conductive or photoconductive insulating layer which is substantially opaque to actinic light and which is capable of conducting electrical charged carriers of one polarity only. Semi-conductive materials, including photoconductors, conduct electricity either in the form of negative electrons or positive holes The photoconductive insulating materials used in xerography generally conduct by both mechanisms although one may predominate. Thus the selenium xerographic layers in common use conduct both holes and electrons although they are better conductors of holes than of electrons. Methods are also known whereby such layers may be formed which are better electron than hole conductors. When such a layer is charged and illuminated the actinic light is absorbed in a very short distance and electron hole pairs are formed near the surface of the layer. Depending upon the polarity of the applied charge one polarity of charged carrier diffuses towards the surface of the layer while the other polarity diffuses away from the surface and it is this latter polarity which is effective in discharging the plate. If, however, the surface of the photoconductive insulating layer comprises a material which is a poor conductor for that polarity of charge carrier which diffuses away from the surface the layer will be effectively insensitive to light. In terms of a specific example a conventional vitreous selenium may be formed on a transparent support layer and coated with a thin layer of selenium which contains deep electron traps and is therefore substantially incapable of conducting electrons. If such a plate is negatively charged and conventionally exposed to light the incident light will be absorbed in the uppermost layer of selenium and will generate electrons and holes therein. The holes will tend to move toward the surface of the selenium but since they are already very close to the surface such movement is ineffective in neutralizing the electrostatic charge on the plate. The electrons are immediately immobilized in the surface layer and therefore they are also unable to contribute towards electrically discharging the plate. Accordingly, such a plate, when negatively charged, is -substantially insensitive to light when conventionally exposed. If, however, the plate is exposed to the transparent support layer electron hole pairs are generated near the interface of the selenium with the transparent support layer and the holes are free to travel `across the photoconductive insulating layer 13 and thus discharge the plate since ordinary selenium of high purity is a good hole conductor. Accordingly such a plate is sensitive to light when exposed through the transparent support layer.

A plate of the foregoing type may be made by placing a tin oxide coated glass member or other suitable transparent support in a vacuum chamber and evaporating high purity selenium onto the glass in accordance with conventional vacuum deposition procedures as are known in connection with the xerographic and other arts. Shortly before the conclusion of the evaporation process a small amount of oxygen is admitted `to the vacuum chamber and becomes incorporated in the surface portions of the selenium layer. It is also possible, and generally preferable, to admit oxygen throughout the evaporation process. Selenium which is contaminated or doped with oxygen is a very poor conductor of electrons. Consequently xerographic plates made according .to the foregoing procedure exhibit the desired properties of light sensitivity when exposed from the back and light insensitivity when exposed from the front. The above plate making procedure is intended to be illustrative only and other materials known -to the semi-conductor art may be used in accordance with the desensitizing principles of the invention.

It is also feasible and within the scope of this invention to form the 'light desensitizing layer in a half-tone pattern.

FIG. 3 illustrates one mode lof utilization of the xerographic plate of FIG. 1. FIG. 3A represents or shows the charging or sensitizing of the plate 10. This 4is acoomplished moving the plate relatively to a conventional corona discharge device 31 which is `connected to a high voltage power supply 32. This is a completely conventional xerographic step except that dark slide 11 remains in position and except that this operation can be carried out 4in normal room light. FIG. 3B shows the exposure of the plate 10 after it has 4been charged. The plate is positioned within a light-tight camera 33, dark slide 16 is removed and the plate is exposed to a suitable original subject 34 by lamps 35 and lens 36 mounted as shown in a light-tight partition 37. Dark slide 16 is then replaced and the plate may tbe removed from the camera. The exposure step is the only one which cannot be carried out in normal room light. The electrostatic latent image |formed in the exposure step is then developed or made visible as shown in FIG. 3C. The plate 10 is positioned at an oblique angle from the horizontal with its lowest end resting in a tray 38. A supply of cascade developer material 39 is then poured over the upper surface of the plate 10, i.e., the surface opposite that of the transparent support layer. As is well known in the xerographic art cascade developer materials generally comprise a mixture of micro -size pigmented toner particles mixed with much larger carrier beads. are commercially available from Xerox Corporation, Rochester, New York. When developer 39 is poured over plate 10some of the toner particles are selectively attracted to plate 10 by the electrostatic latent image -thereon and .form a visible pattern corresponding to the original subject matter 34 of FIG. 3B. Various xerographic developer methods and materials are known' be` Such materials sides the one just described and may be used in conjunction with the present invention. The described method, however, is a particularly simple and effective procedure which has not `heretofore been practical because it was necessary that the procedure be carried out in a dark room or the like. In accordance with the present invention, however, this simplified development method attains practicality because it can be performed in ordinary room light and requires no dark room, dark box or other equipment :other than the simple elements shown `in FIG. 3C.

The image on plate 1l) may then be transferred to a shee-t of paper 40 or the like as shown in FIG. 3D. Paper 40 is laid over the image on plate 10 which is then passed beneath a corona discharge device 31 which may be the same device as shown in FIG. 3A. When paper 40 is removed from plate 10 itis found to carry the toner image with it. The transfer procedure of FIG. 3E may also be used following that of FIG. 3D .or as a substitute therefor. In accordance with FIG. 3E paper 40 is first removed from plate in connection with a roller 41 which is rolled across paper 40. Thus any given portion of paper 40 is in contact `with roller 41 as it leaves plate 10 and is notv separated from roller 41 until after it has been separated from plate 10. Roller 41 is at least slightly electrically conductive and is 4connect-ed t-o a power supply 42 which supplies a voltage generally in the range of 500 to 900 volts although the voltage may also -be zero. When this transfer procedure is followed the developed image adheres to and is transferred to paper V4ta while the electrostatic latent image on plate 1t) is not destroyed and remains substantially as it was prior to development. Accordingly the plate may again be developed as shown in FIG. 3C and the newly developed image may again be transferred as shown in FIGS. 3D and 3E or FIG. 3E alone. This procedure, which is more fully described in U.S. Patent 2,951,443 may be repeated many times to yform many copies of an original subject with only one charging of plate 10 and only one exposure of the plate to the original subject.V While the basic .procedure is known and is fully described in the above referenced patent lit could not previously be carried out in the manner shown herein because all the steps had to be carried out in darkness or at least under safe light illumination. The multicopy procedure of the patent avoids destroying the electrostatic latent image in the transfer step but does not prevent destruction of the image if the plate is permitted to -be exposed to room light. In accordance with the present invention, however, a multicopy process may be carried ou-t as illustrated in normal room light and accordingly becomes feasible for use in small ofilces and the like which have only simple manual xerographic equipment where transfer must be carried out manually and in an exposed location.

FIG. 4 is a schematic representation of a simple and novel xerographic copying apparatus particularly adapted for use with the xerographic plates of the present invention. The apparatus includes a cubical box or a similar light-tight enclosure 50 which is mounted in brackets 51 for Irotation -about a horizontal axis. One of the faces of the box parallel to the axis of rotation incorporates a hinged copy board 52 as illustrated. A pair of fluorescent lamps 53 is posi-tioned within box 50 and adapted to il? luminate copy board 52. Also positioned within box 50 are a reflecting prism 54 and lens 55 which are adapted to project an image o-f any material on copy board 52 onto xerographic plate 10, which is mounted in one of the faces of box 50 which is parallel to the axis of rotation and also adjacent to the face carrying copy hoard 52. Plate 10 may be the same structure as shown in FIG. l land is mounted with its transparent support layer facing into the box. Plate holder 15, illustrated in F-IG. l, is not required with this form of apparatus and is preferably omitted. Positioned above plate 10 and mounted on hinges 56 is a charging grid 57 which is connected to a high voltage .power supply 32. Charging grid 57, which may #be of the known type 2,932,742 comprises a Igrid of tine wires covering the entire operative area of plate 10 and is adapted to uniformly electrostatically change plate 10 without movement relative thereto. The yface of box 50 which carries plate 10 is also provided at `its outer ledge with a set olf projecting lips 58 which are adapted to retain a quantity of xerographic developer 39. In operation a sheet of printed matter or other original subject to be `copied is placed on copy board 52 which is then swung up until it is flush with box 50, the box meanwhile being rotated to keep the copy yboard in an approximately horizontal position, unless clips or the like, not illustrated, are provided to keep the original subject from sliding on the copy board. Power supply 32 is then briefly energized to put an electrostatic charge on pla-te 10 and lamps 53 are next briefly energized to project a light ima-ge of the original subject on copy board 52 onto plate 10. Charging grid 57 is then swung away from plate 10 and -box 50 is slowly rocked back and forth on its axis about the position in which plate 10 is uppermost, thereby causing developer 39 to flow back and yfor-th over plate 10 and form a developed image thereon. The developed image may be transferred to a sheet of paper or the like by placing the paper on plate 10 and repositioning and reenergizing charging grid 57, after which paper may be removed bearing a right reading reproduction of the original subject matter. Transfer may also be carried ou-t according to the procedure described in connect-ion with FIG. 3E, in which case box 50 may again be rocked to develop the image on plate 10 and in this manner numerous copies may be made of the original subject without recharging or reexposing plate 10. It will be noted that this apparatus -functions in ordinary room light and therefore only becomes practical in conjunction `with xerographic plates which are insensitive to light at the surface on which the image is developed.

FIG. 5 shows a portion of the apparatus of FIG. 4 incorporating certain modifications. Plate 10 is mounted on a set of arms 58 and may be moved from the illustrated position to the one indicated by dotted llilies by op-V erating handle 59 which is preferably positioned on the outside of box S0. A corona charging device 31 of conventional design is connected to high voltage power supply 32 and is positioned above the illustrated position of plate 10 but below the alternate position thereof. Charging device 31 slides upon guide bars 60 and is mechanically attached to a rod 61 which extends through the wall of box 50 and permits the charging device to be moved from outside the box. Bars and rod 61 are so positioned that when the charging device 31 is withdrawn to one side plate 10 can be moved from the illustrated position to the alternate position in which it is sealed against the side of box 50 by rubber gaskets 52. Positioned above plate 10 on the outside of box 50 is a pair of guide bars 53 on which -is slidably mounted an electrically conductive resilient roller 41 connected to a power supply 42. Roller 41 is adapted to be manually pressed in engagement with plate 10 by a handle 64. In operating this machine the original copy is inserted as in FIG. 5, power supply 32 is energized and rod 61 is operated to move charging device 31 across plate 10, after which power supply 32 is turned oil and handle 59 operated to move plate 10 against gaskets 62. Development is then carried out exactly as described in connection with FIG. 4. Transfer is carried out by laying a piece of paper or the like against plate 10, energizing power supply 42 to apply a voltage'of from 500 to 900 volts to roller 41, and rolling the roller across the paper with the aid of handle 64, while simultaneously peeling the paper away from plate 10 as shown in FIG. 3E. Since the transfer procedure does not destroy the electrostatic latent image on plate 10, many copies may be made in rapid succession by single exposure by the procedure already described. It will be noted that in this form of apparatus the chargdescribed in U.S. Patent 7 ing equipment is housed wit/1in box 50, thereby allowing greater freedom in choosing and operating the transfer apparatus, which may accordingly be quite distinct from the charging apparatus as illustrated, for example, in this figure.

The improved plates and procedures of the present invention are also adapted to the recording of X-ray images r other images of penetrating radiation as well as to the recording of visible light images. It is well known that Xerographic plates are sensitive to X-rays, gamma rays, beta rays and like forms of high energy penetrating radiation as well as to low energy non-penetrating radiation such as visible or ultra-violet light. However, this X-ray sensitivity has heretofore been coupled with a sensitivity to visible light, and it has accordingly been necessary to keep the photoconductive insulating layer of an X-ray recording plate shielded from visible light from a time prior to charging to a time subsequent to development. This considerably complicates the making of xerographic X-ray images because the same photoconductive insulating layer must be made accessible for charging and development. Plates according to the present invention, however, are insensitive to light impinging on the exposed surface of the photoconductive insulating layer, and it is thus necessary y only to prevent visible light from reaching the opposite side of the photoconductive layer in order to render the plate insensitive to visible light while still remaining sensitive to X-rays. This can readily be accomplished by leaving dark slide 16 of FIG. 1 in position at all times gr by coating support layer 11 with black paint or other opaque material or by making support layer 11 of a material such as aluminum which is opaque to visible light while preferably transparent to X-rays. Such a plate may be charged, exposed to a pattern of X-rays and developed in normal room light and without any special apparatus or manipulations being required to protect the plate from ambient light. Exposure to the X-ray pattern may be made to the front of the plate, or where support layer 11 is sufficiently transparent to penetrating radiation it may also be madeto the back of the plate. The present invention thus provides for X-ray xerography the same advantagesv and simplications as it provides for visible light xerography. In addition it minimizes the undesirable effect known as fatigue which is otherwise caused by excessive exposure of an X-ray or other xerographic plate to ambient light.

While the present invention, as to its objects and advantages, has been described herein as carried out in specific embodiments thereof, it is not intended to be limited thereby, but it is intended to cover the invention broadly within the spirit and scope of the appended claims.

What is claimed is:

1. A xerographic plate comprising an electrically conductive transparent support layer, a layer of photoconductive insulating material coated thereover, and coated over said layer of photoconductive insulating material a'layer opaque to activating radiation for said photoconductive insulating material.

2. A xerographic plate according to claim 1 in which the light desensitizing layer comprises a continuous layer of electrically insulating resin including a light absorbing dye.

3. A xerographic plate according to claim 1 in which the layer opaque to activating radiation comprises a semiconductive material having substantially no conductivity for charge carriers of one polarity.

4, A xerographic plate comprising in sequence and combination an electrically conductive transparent support layer, a layer of photoconductive insulating vitreous selenium, and a thin continuous layer of cellulose acetate butyrate including nigrosine dye.

5. A xerographic plate comprising in sequence and combination an electrically conductive transparent support layer, a layer of photoconductive insulating vitreous selenium, and a thin layer of selenium containing suicient oxygen to render said selenium substantially incapable of transporting negative electric charge, said thin layer being substantially opaque to actinic light for vitreous selenium.

6. The method of forming on a surface an electrostatic latent image which is resistant to destruction by the action of light directed at said surface comprising forming a photoconductive insulating layer over an electrically conductive transparent support layer, forming over said photoconductive insulating layer, a layer opaque to activating radiation for said photoconductive insulating layer, and applying a pattern of light and shallow to said transparent support layer while maintaining an electric field through said photoconductive insulating layer.

References Cited by the Examiner UNITED STATES PATENTS 2,901,348 8/1959 Dessauer et al. 96-1 3,124,456 3/1964 Moore 96-1 NORMAN G. TORCHIN, Primary Examiner.

ALEXANDER D. RICCI, Examiner.

A. L. LIBERMAN, C. E. VAN HORN,

Assistant Examiners. 

1. A XEROGRAPHIC PLATE COMPRISING AN ELECTRICALLY CONDUCTIVE TRANSPARENT SUPPORT LAYER, A LAYER OF PHOTOCONDUCTIVE INSULATING MATERIAL COATED THEREOVER, AND COATED OVER SAID LAYER OF PHOTOCONDUCTIVE INSULATING MATERIAL A LAYER OPAQUE TO ACTIVATING RADIATION FOR SAID PHOTOCONDUCTIVE INSULATING MATERIAL. 